add pavement script to help with dev, refs #71

requires pavement, run paver help to get commands.

note - changes external libraries directory to ext-libs to prevent python path confusion (since ext-libs is not a valid package directory)

also supports adding 'source' requirements - git checkouts, like for gsconfig

replaces the install.py and package.py scripts, too

currently all tests and deps are packaged though this could be adjusted